The present invention relates to magnetic storage media for magnetically recording and reproducing information in a recording domain by heating the domain to a high temperature using a heat source, and also to heat assisted recording and reproduction methods applied to such media.
Heat assisted magnetic recording and reproduction schemes have recently been developed to realize high density storage as a fusion of optical technology and magnetic recording and reproduction technology. Japanese Laid-Open Patent Application No. 4-176034/1992 (Tokukaihei 4-176034, corresponding to U.S. Pat. No. 5,656,385 dated Aug. 12, 1997), for example, discloses a magnetic storage medium made of a ferromagnetic material having a compensation temperature set substantially to room temperature, as well as a heat assisted magnetic recording and reproduction scheme using laser light for such a medium.
In the heat assisted magnetic recording and reproduction scheme, information is recorded in a recording domain of a magnetic storage medium by applying an external magnetic field from a recording magnetic head while heating the recording domain to a high temperature using laser light to reduce the coercive force of the recording domain. Meanwhile, according to the scheme, information is reproduced by, again, heating the recording domain to a high temperature with laser light to amplify the strength of residual magnetization in the recording domain, and detecting the magnetic flux generated by the residual magnetization using a reproduction magnetic head.
In the foregoing scheme, the domains where temperature remains at room temperature without being elevated to high temperatures with laser light, have residual magnetization that is ignorably small. Therefore,the crosstalk from adjacent tracks can be restrained to substantially low levels even with a gap width, i.e., a width of the reproduction magnetic head measured perpendicular to the track direction, that is larger than the pitch of the track where information is stored, enabling reproduction of information from a high density storage medium.
Meanwhile, Japanese Laid-Open Patent Application No. 4-95201/1992 (Tokukaihei 4-95201, corresponding to Japanese Patent 2636957, Date of Patent Apr. 25, 1997) discloses a heat assisted magnetic reproduction method whereby signals from adjacent tracks are reduced to restrain crosstalk by heating both sides of the track to be reproduced to high temperatures using a light beam so as to elevate the temperature thereof to a neighborhood of the magnetic compensation temperature at which the residual magnetization is zero.
According to conventionally suggested heat assisted reproduction methods, information is retrieved either by amplifying magnetization by way of heating a reproduction domain or by reducing magnetization to restrain crosstalk by way of heating a neighborhood of a reproduction domain.
Therefore, heat works on only limited areas: either a reproduction track domain or two adjacent tracks of a reproduction track. Either way, information is reproduced using difference in the strength of residual magnetization between domains where temperature is elevated and where temperature is not elevated.
The magnetic compensation temperature of a ferrimagnetic material storage medium is highly susceptible to the composition of the storage medium; a small variation in the composition causes a great change in the magnetic compensation temperature.
For example, an experiment conducted by the inventors of the present invention on a magnetic film of a TbFeCo alloy showed that a fluctuation as small as a few percentage points in the composition ratio of the Tb and the FeCo causes the magnetic compensation temperature to change a few dozen degrees centigrade.
FIG. 26 shows the dependency on the composition ratio, wherein the composition ratio of Tb and FeCo is plotted along the axis of ordinates and the magnetic compensation temperature (referred to as compensation temperature in the figure) is plotted along the axis of abscissas. In this example, a 1% change in the composition ratio caused a 20xc2x0 C. change in the magnetic compensation temperature.
Consequently, irregularities in the manufacturing process of magnetic storage media constitute an obstacle in the precise setting of the magnetic compensation temperature. Further, considering the actual operational conditions for the recording and reproduction system, since ambient temperature supposedly differs wildly depending on operational environment, the foregoing heat assisted reproduction has trouble in the accurate setting of temperature in the non-elevated temperature domain of the storage medium.
As described in the foregoing, the conventionally suggested heat assisted recording and reproduction has following problems: (1) It is difficult to precisely specify the magnetic compensation temperature of the storage medium and the temperature of the non-elevated temperature domain; (2) Occurrence of crosstalk is inevitable, and (3) Stability in recording and reproduction is low.
With the foregoing problems taken into consideration, the present invention has an object to offer a heat assisted recording and reproduction method that is stable against, and is hardly affected by, irregularities in the magnetic compensation temperature that occur in the manufacturing process of a magnetic storage medium due to irregularities in the composition, and variations in the temperature of the storage medium caused by ambient temperature at the magnetic storage medium operates.
The present invention has another object to offer a magnetic storage medium used together with a heat source, such as a head employing magneto-resistance effect, as well as to offer a heat assisted recording and reproduction method applied to such a magnetic storage medium, the magnetic storage medium being suited to reproduce information stored in high density with crosstalk occurring due to irregularities in the temperature of the medium only in a restrained manner and hence being capable of producing a good S/N ratio, even if a heat assisted magnetic recording and reproduction method is applied.
To solve the foregoing problems, a magnetic storage medium of the present invention includes a storage layer for use in heat assisted recording and reproduction whereby information is magnetically recorded and reproduced by heating a recording domain, wherein saturated magnetization of the storage layer has a maximum value at a temperature between a magnetic compensation temperature and a Curie point, the temperature being specified in a range from 150xc2x0 C. to 250xc2x0 C., and the magnetic compensation temperature of the storage layer is specified higher than room temperature.
In the arrangement, in the magnetic storage medium, the temperature at which the saturated magnetization of the storage layer takes its maximum value between the magnetic compensation temperature and the Curie point is specified in a range from 150xc2x0 C. to 250xc2x0 C.; therefore, in the recording domain heated to a neighborhood of the temperature at which the saturated magnetization takes its maximum value, the residual magnetization of the recording domain can be increased, and the information can be reproduced satisfactorily from the recording domain of the storage layer.
Here, for example, even if the head acts as a heat source causing the ambient temperature of the storage layer to exceed room temperature, such as in a case where a head exhibiting a magneto-resistance effect is used for reproduction, since the magnetic compensation temperature of the storage layer can be specified according to the ambient temperature that is higher than room temperature, the temperatures of the recording domains, except that of a recording domain heated for the purpose of reproduction, are equal to the ambient temperature, i.e., equal to the magnetic compensation temperature, and the magnetization is substantially zero. As a result, in the medium, the recording domain to be reproduced is protected from negative effects of magnetization of the other recording domains during reproduction, restraining crosstalk during reproduction.
Further, in the arrangement, by elevating the temperature of the recording domain in the storage layer to a neighborhood of the Curie point, the coercive force of the recording domain can be reduced, and magnetization of the recording domains can be similarly rendered substantially zero except for the aforementioned recording domain, and information can be stably recorded in the recording domain using an external magnetic field that is in accordance with the information.
Further, in the arrangement, the magnetic compensation temperature can be specified to differ from room temperature, for example, higher than room temperature; therefore, when recorded information is to be reproduced, the residual magnetization of the recording domains outside the read-out recording domain (hereinafter, will be simply referred to as the read-out domain) can be minimized by controlling the temperature to which heating means heats the storage layer despite possible occurrence of irregularities in the magnetic compensation temperature. Consequently the arrangement offers an advantage that the S/N ratio of a reproduction signal improves. The heating means may be a head exhibiting magneto-resistance effect which in practice doubles as a heat source.
In the magnetic storage medium, the magnetic compensation temperature of the recording domain may be specified according to a temperature to which the recording layer is heated by the head generating heat based on reproduction of information by magneto-resistance effect.
In the arrangement, even if the storage medium is heated to an elevated temperature by the heat generated by the magneto-resistance effect of the head, since the magnetic compensation temperature is specified higher by at least that elevation in temperature, the residual magnetization can be minimized in the recording domains outside the read-out domain, and a reproduction signal is obtained with a good S/N ratio and restrained crosstalk, which meets the object of the heat assisted reproduction.
To solve the foregoing problems, a heat assisted recording and reproduction method in accordance with the present invention includes the steps of:
preparing a storage medium including as a storage layer a magnetic film exhibiting a magnetic compensation temperature higher than room temperature;
specifying a plurality of heated domains on the storage medium according to the magnetic compensation temperature, the plurality of heated domains each having a different target temperature from the other(s); and
recording and/or reproducing information in the storage layer in one of the plurality of heated domains.
As a result, according to the method, the storage medium can be specified to include a plurality of heated domains each having a different target temperature from the other(s) such that a first heated domain in the heated domain is heated to the magnetic compensation temperature and also that a second heated domain, which is not the first heated domain in the heated domain, is heated to a temperature different from the magnetic compensation temperature, for example, higher than the magnetic compensation temperature.
This allows, with the method, the coercive force and the residual magnetization of the storage layer in the second heated domain to be reduced and increased respectively, and therefore enables information to stably be recorded and/or reproduced on the storage layer in the second heated domain.
Also, according to the method, by providing the storage layer of the first heated domain outside the domains where actual recording or reproduction of information takes place and also by heating the storage layer to the magnetic compensation temperature, the residual magnetization can be reduced, for example, to substantially zero; therefore the second heated domain where information is recorded and reproduced is protected from magnetic effects.
Consequently, according to the method, the temperature of the first heated domain can be specified according to the magnetic compensation temperature of the storage layer; therefore generation of unnecessary magnetization by irregularities in the magnetic compensation temperature of the storage layer and variations in ambient temperature can be restrained, allowing information to be reproduced more stably.
According to the method, the temperature of the heated domain when information is recorded and the temperature of the heated domain when information is reproduced may be specified equal to each other, and alternatively, different from each other: for example, the temperature of the heated domain when information is recorded may be specified higher than the temperature of the heated domain when information is reproduced. If the temperatures are specified equal to each other, heat control can be facilitated. In contrast, if the temperature of the heated domain when information is recorded is specified higher than the temperature of the heated domain when information is reproduced, the heated domain in recording has a reduced coercive force and thus allows a weaker external magnetic field to perform recording, facilitating the recording process.
Another heat assisted recording and reproduction method in accordance with the present invention is such that when information recorded in the recording domain of the magnetic storage medium is reproduced using a head exhibiting magneto-resistance effect, the bias current applied to the head is altered according to the temperature of the recording domain.
According to the method, when recorded information is reproduced from the magnetic storage medium using a head exhibiting magneto-resistance effect, heat generation by the head can be controlled by adjusting the bias current applied to the head according to the temperature of the recording domain, thereby causing the temperature of the recording domains, except for the recording domain to be reproduced, in a neighborhood of the head to be rendered closer to the magnetic compensation temperature of the recording domain.
Consequently, according to the method, even if there occurs a deviation in the magnetic compensation temperature due to irregularities in composition of the storage domain or a variation in the heating amount due to a change in the linear velocity of the head, the residual magnetization can be minimized in the recording domains outside the recording domain to be reproduced, and crosstalk is restrained; therefore, the S/N ratio in the reproduction signal can be maintained at a satisfactory level.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.